Specific cell killing can be essential for the successful treatment of a variety of diseases in mammalian subjects. Typical examples of this are in the treatment of malignant diseases such as sarcomas and carcinomas. However the selective elimination of certain cell types can also play a key role in the treatment of many other diseases, especially immunological, hyperplastic and/or other neoplastic diseases.
The most common methods of selective treatment are currently surgery, chemotherapy and external beam irradiation. Targeted endo-radionuclide therapy is, however, a promising and developing area with the potential to deliver highly cytotoxic radiation to unwanted cell types. The most common forms of radiopharmaceutical currently authorised for use in humans employ beta-emitting and/or gamma-emitting radionuclides. There has, however, been a recent surge in interest in the use of alpha-emitting radionuclides in therapy because of their potential for more specific cell killing. One alpha-emitting nuclide in particular, radium-223 (223Ra) has proven remarkably effective, particularly for the treatment of diseases associated with the bone and bone-surface. Additional alpha-emitters are also being actively investigated and one isotope of particular interest is the alpha-emitter thorium-227.
The radiation range of typical alpha emitters in physiological surroundings is generally less than 100 micrometers, the equivalent of only a few cell diameters. This makes these nuclei well suited for the treatment of tumours, including micrometastases, because little of the radiated energy will pass beyond the target cells and thus damage to surrounding healthy tissue might be minimised (see Feinendegen et al., Radiat Res 148:195-201 (1997)). In contrast, a beta particle has a range of 1 mm or more in water (see Wilbur, Antibody Immunocon Radiopharm 4: 85-96 (1991)).
The energy of alpha-particle radiation is high compared to beta particles, gamma rays and X-rays, typically being 5-8 MeV, or 5 to 10 times that of a beta particle and 20 or more times the energy of a gamma ray. Thus, this deposition of a large amount of energy over a very short distance gives α-radiation an exceptionally high linear energy transfer (LET), high relative biological efficacy (RBE) and low oxygen enhancement ratio (OER) compared to gamma and beta radiation (see Hall, “Radiobiology for the radiologist”, Fifth edition, Lippincott Williams & Wilkins, Philadelphia Pa., USA, 2000). These properties explain the exceptional cytotoxicity of alpha emitting radionuclides and also impose stringent demands on the level of purity required where an isotope is to be administered internally. This is especially the case where any contaminants may also be alpha-emitters, since these can potentially be retained in the body and cause significant damage. Radiochemical purity should be as high as reasonably feasible and contamination with non-targeted radionuclides should be minimised, particularly where the contaminant is an alpha-emitter.
The radioactive decay chain from 227Ac, generates 227Th and then leads to 223Ra and further radioactive isotopes. The first three isotopes in this chain are shown in FIG. 6. The table shows the element, molecular weight (Mw), decay mode (mode) and Half-life (in years (y) or days (d)) for 227Th and the isotopes preceding and following it. Preparation of 227Th can begin from 227Ac, which is itself found only in traces in uranium ores, being part of the natural decay chain originating at 235U. One ton of uranium ore contains about a tenth of a gram of actinium and thus although 227Ac is found naturally, it is more commonly made by the neutron irradiation of 226Ra in a nuclear reactor.
It can be seen from this illustration that 227Ac, with a half-life of over 20 years, is a very dangerous potential contaminant with regard to preparing 227Th from the above decay chain for pharmaceutical use. Even once the 227Ac is removed or reduced to a safe level, however, 227Th will continue to decay to 223Ra with a half-life of just under 19 days. Since 223Ra is an alkaline earth metal it will not easily be coordinated by ligands designed for thorium or other actinides. This 223Ra then forms the beginning of a potentially uncontrolled (untargeted) decay chain including 4 alpha-decays and 2 beta-decays before reaching stable 207Pb. These are illustrated in the table below:
Nuclide227Th223Ra219Rn215Po211Pb211Bi207Tl207pb½-life18.7 d11.4 d4.0 s1.8 ms36.1 m2.2 m4.8 mstableα-energy/6.155.646.757.396.55MeVβ-energy1.371.42(max)/MeVEnergy %17.516.019.121.03.918.64.0
It is evident from the above two decay tables that 223Ra cannot be entirely eliminated from any preparation of 227Th because the latter will constantly be decaying and generating the former. It is clear, however, that more than 25 MeV in radiated energy will be released from the decay of each 223Ra nucleus administered to a patient, before that nucleus reaches a stable isotope. It is also probable that such 223Ra will not be bound and targeted by the systems of chelation and specific binding designed to transport 227Th to its site of action, due to the differing chemical nature of the two elements. Therefore, for the purpose of targeted cell killing, maximising the therapeutic effect and minimising side-effects, it is important to have control over the level of 223Ra in any 227Th preparation prior to administration.
Separation of 227Th from 223Ra could be carried out quickly and conveniently in a radiological laboratory. However, this would not achieve the desired result effectively because the resulting purified 227Th must then be transported to the site of administration.
In view of the above, it would be a considerable advantage to provide a method of purifying 227Th from contaminant 223Ra which could be carried out at or close to the point-of-care, at or shortly before the time of administration utilising a simple method that would not require extensive training and experience to carry out. It would be an advantage if the use of strong mineral acids and/or strong bases could be avoided from a safety and handling point of view. This applies particularly if the reagents used are suitable for direct use in the final drug product. It would also be an advantage if small volumes could be used to ease handling and reduce the volume of contaminated waste. It would be a further advantage if this method could be implemented with a simple group of reagents and items of apparatus, which could be supplied for such a contemporaneous preparation, optionally in the form of a kit.
Previously known preparations for 227Th have generally been for laboratory use and/or not tested for purity to pharmaceutical standards. In WO2004/091668, for example, 227Th was prepared by anion exchange from a single column and used for experimental purposes without validation of the purity. The primary aim of separation in most preparative methods for 227Th has been the removal of the long-lived 227Ac parent isotope. Methods have not previously been devised or optimised for removal of 223Ra which has grown-in in a 227Th sample previously purified from 227Ac.